In ducted fan gas turbine engines, air entering the intake of the engine is accelerated by a fan to produce two air flows: a first air flow which enters a core engine, and a second air flow which passes through a bypass duct to provide propulsive thrust. A fan case surrounds the fan and extends rearwardly therefrom to define an outer wall of the bypass duct. An inner wall of the bypass duct is defined by the outer skin of the core engine.
Between the inner and outer walls of the bypass duct, rearwards of the fan, it is known conventionally to provide A-frame support structures which reinforce the fan case and transmit loads from the core engine to the fan case.
FIG. 1 shows schematically a transverse cross-section through an engine which has two diametrically opposing A-frame support structures 101. Each support structure comprises two support struts 102 which extend from spaced apart locations on the core engine 104 to meet at the fan case 106.
However, as shown in FIG. 2, under a torque load T the support structures 101 can distort the shape of the fan case 106 (as indicated by the dashed lines), resulting in excessive fan tip clearances and associated aerodynamic losses. In addition, the junction of the support struts 102 within the bypass duct, presents an undesirable aerodynamic blockage to the airflow within the bypass duct producing further aerodynamic losses.
One approach to reduce the distortion is to stiffen the fan case with, for example, a stiffening flange. However, this increases weight, and does not address the undesirable blockage to the airflow within the bypass duct.
An alternative approach is to space the support struts 102 apart at the fan case 106, as shown in FIG. 3, with local reinforcements 108 spreading loads into a stiffener flange 110. This approach reduces the aerodynamic losses due to the distortion and aerodynamic losses associated with the support struts due to no longer having the junction of the struts within the bypass duct. However, the struts transmit loads inefficiently into the structure, so requiring the local reinforcements and the stiffener flange which can increase the weight of the structure.